1. A Star Destroyer exploding in Star Wars: X-Wing. Also the mission debriefing describing your purple-heart-worthy defense of a Rebel Frigate from one of the aforementioned behemoths.
2. sigh, a scarab exploding in Halo 3. Even better because it killed my little brother. Screw that noob.
3. The very first mission of CoD4 where you see Captain Price throw his cigarette out of the chopper, then land on a cargo ship. Better than any nuke.
4. The effects on Lost Planet. The blurring for swift movement and the quick flash of an explosion, along with the smoke make that game the prettiest in my opinion.
5. Star Wars: Mysteries of the Sith, dropping an A-Wing on a couple of unsuspecting Stormtroopers. +1,000,000% awesomeness if you can get on top of the A-Wing before it drops, pull out the lightsaber, and drop down on them too.
There's more, but I wont remember them until after I post. Figures.